[FPSPACE] Soyuz DM roll during ballistic entry profile

[Geert Sassen]

The method to 'steer' the DM during re-entry using only the roll 
thrusters and the offset CG in itself is not 'unique', it was used also 
during most Gemini landings and as far as I know also during Apollo. 
Mercury, as well as Vostok and Voschod used a pure ballistic reentry 
(Vostok/Voschod did not even have thrusters on its descent module and 
relied purely on aerodynamics). The Shuttle offcourse has its own, 
unique, method.

What is 'special' and very ingenious about the soyuz DM is that its 
shape has only one 'stable' aerodynamic position: with the headshield 
'down'. A LOT of design effort and mathematics was put into finding the 
correct shape for the soyuz DM and this payed off during various 
emergency's as we have seen now once again. The Apollo (and Gemini??) CM 
due to its shape had TWO stable positions, one with headshield down, the 
other with headshield 'up', in other words it would not automatically 
turn to the correct attitude if you drove it 'upside down' into the 
admosphere, as Soyuz has now once again demonstrated it does. If on 
Apollo the CM failed to separate from the SM prior re-entry you 
definitely had a very bad day...

However, no matter how ingenious the design, all of this should never be 
taken for granted as a 'standard feature', my car is also designed in 
such a way that I'll probably survive when hitting a wall at 50 km/hr 
but that doesn't mean this is my prefered method of parking...

Javier Casado wrote:
> Absolutely clear now, thanks to both, Geert and Antonin.
>
> In fact, as I see, the complication is not in the maneuver to get this
> precession movement, but in the capsule design. What they indirectly achieve
> with that CG offset is (in addition to the proper "natural" attitude for
> reentry) to get a main inertia axis for the vehicle that is not coincident
> with its simmetry axis. This way, a simple firing of the "roll" thrusters
> produces a precession movement instead of pure roll. Ingenious.
>
> It is amazing to me how those old designers did so much with so little.
> Today, virtually no designer could calculate an inertia axis without CAD!
>
> Regards,
>
> Javier